1. Field of the Invention
This invention relates to inspection systems and, more particularly, to inspection systems and methods for reducing speckle noise in images.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during the manufacturing process to detect defects on wafers, promoting higher yield in the manufacturing process, and thus, higher profits. Inspection has always played an important role in the fabrication of semiconductor devices. However, the performance requirements of inspection systems continues to increase, as the dimensions of the semiconductor devices decrease. In comparison to previous systems, today's inspection systems require significantly higher resolution and sensitivity for detecting the small sized defects, which occur on advanced semiconductor wafers.
One way to increase the resolution of an optical inspection system is to decrease the wavelength at which the system can operate. For instance, resolution is defined as: Resolution=λ/n(NA), where λ is the wavelength, n is the index of refraction, and NA is the numerical aperture of the optical system at the object. Therefore, resolution increases as wavelength decreases. In some systems, however, steps taken to increase resolution have an adverse affect on the sensitivity of the system.
For example, there are generally two types of light sources available for use in an optical inspection system. In some cases, an incoherent light source (such as, e.g., an arc lamp, a tungsten incandescent lamp, a deuterium lamp, and a light-emitting diode) may be used to illuminate the semiconductor wafer of specimen. However, most incoherent light sources are incapable of producing light with sufficient brightness as the wavelength of the inspection system is decreased (e.g., to increase resolution). The relative brightness of the light source affects defect sensitivity by affecting the signal-to-noise ratio of the output signals generated by the inspection system. If the brightness is too low, the signal-to-noise ratio may be too low for accurate defect detection. In some cases, the inspection throughput may be reduced to allow enough light to be collected. Obviously, reduced throughput is highly undesirable for inspection.
Therefore, a laser light source is often used to generate brighter light at shorter wavelengths. However, laser light sources produce coherent light, which is undesirable for inspection for many reasons. For example, coherent light tends to introduce speckle and/or ringing into the inspection images generated by the image detector. Speckle decreases the sensitivity of the inspection system by decreasing the signal-to-noise ratio of the output signals generated by the inspection system. Ringing introduces artifacts into the inspection images, which reduce sensitivity and make it difficult to detect defects. Many illumination systems have been designed to mitigate the affects of speckle and/or ringing.
In some cases, speckle and/or ringing may be improved by reducing the spatial coherence of the laser light used to illuminate the specimen. One technique for providing partially incoherent laser light involves the use of a rotating diffuser. The diffuser is arranged within the path of the incident laser beam and rotated to introduce random phase variations into the beam.
As the diffuser rotates, multiple images of the specimen are collected from independent views or perspectives by an image detector (e.g., a CCD or TDI detector). The multiple images are averaged over the integration time of the image detector to reduce speckle and ringing. For example, the mask inspector 5xx provided by KLA-Tencor Corp. of San Jose, Calif. includes a diffuser, which may be configured for averaging approximately 250,000 images over the integration time of the image detector (e.g., about 0.5-1.0 msec). Since the noise reduction factor is the square root of the number of images averaged, the diffuser may provide a noise reduction factor of about 500 times.
In some cases, further noise reduction may be needed as device dimensions continue to shrink to smaller and smaller sizes. One obvious way to reduce noise is to increase the number of images averaged. However, in order to do so, one would have to either: (a) increase the integration time of the detector, or (b) increase the rotational rate of the diffuser. Increasing the integration time of the detector is usually undesirable because it reduces the throughput of the inspection system. However, increasing the rotational rate of the detector may not be desirable, either. For example, the diffuser included within the mask inspector 5xx currently rotates at about 20,000 rpm. Increasing the rotational rate to higher rotational speeds may lead to stress, vibrations and potential damage.
Therefore, a need exists for an improved inspection system and method for reducing speckle in images obtained using coherent illumination. Preferably, the system and method would minimize speckle and improve defect sensitivity, while maintaining inspection throughput and avoiding undue stress on rotational system components.